1. Field of the Invention
The present invention relates to a joint head of a thick steel tube having a relatively small diameter for a high-pressure metal piping such as a high-pressure fuel injection tube to be arranged for frequent uses as a feed passage of light oil or a corrosive fuel such as dimethyl ether or methanol in a Diesel internal combustion engine, for example, and a common rail such as a main rail or a block rail formed of a high-pressure fuel manifold to which the high-pressure metal piping is jointed so that it may act as a pressure accumulator.
2. Description of the Prior Art
As a mating member of the prior art to which a high-pressure metal piping or the like is to be jointed, such as a common rail, there is the type, as shown in FIG. 9, in which a branch hole 11-2 having a circular section leading to an internal communication passage 11-1 formed in the circumferential wall of a main rail 11 having a circular section is formed into a receiving seat 11-3 opened to the outside. By using a ring-shaped joint fixture 13 enclosing the outer circumference of the main rail 11 in the vicinity of the receiving seat 11-3, a pressure seat 12-3, as formed of a joint head 12-2 of a branch tube 12 which has a passage 12-1 extending therethrough and which is radially enlarged by the buckling to form a frusto-conical or spherical end portion, is brought into abutment against the receiving seat 11-3. The branch tube 12 and the main rail 11 are jointed by the pressure below the neck of the joint head 12-2 by fastening a fastening nut 14, as built in advance on the branch tube 12, into a threaded wall 13-1 protruded from the joint fixture 13. In another type, as shown in FIG. 10, the ring-shaped joint fixture 13 is replaced by a cylindrical sleeve nipple 13a, which is welded or soldered directly to the outer circumferential wall of the main rail 11. The pressure seat 12-3, as formed on the joint head 12-2 of the branch tube 12, is brought into abutment against the receiving seat 11-3 of the main rail 11, and the joint is made by fastening the fastening nut 14 into the sleeve nipple 13a. In another known type, as shown in FIG. 11, a boss portion 11-4, as made integral with the main rail 11, has the branch hole 11-2 communicating with the communication passage 11-1 and the r 11-3 opened to the outside. A washer 15-2, as formed to have a section of general parallelogram, is fastened onto the back face of the frusto-conical pressure seat 12-3, as formed by the joint head 12-2 of the branch tube 12, by a fastening nut 16 onto the boss portion 11-4 thereby to make the fitting joint. Here, reference numeral 15-1 designate a sleeve washer.
In the prior art, however, the joint head 12-2 for the high-pressure metal piping as the branch tube 12, as used in the common rail of this kind, is formed into the frusto-conical shape (or frusto-arcuate shape) forming its outer circumference into the pressure seat 12-3, as shown in detailed in FIG. 12. The joint head 12-2 is ordinarily formed by buckling the end portion of a thick steel tube 12' from the outside in the axial direction by a punch member. The joint head 12-2, as formed by this method, is constructed to have an annular recess (or pocket) 12-4 therein as a result of the bulge of the circumferential wall, as accompanying the buckling of the pressure, and is used with the annular recess 12-4. However, this use is accompanied by the following problems. Due to the fluid pressure violently fluctuating at a high level and at a high speed during the use, there arises a cavitation/erosion or corrosion (as will be called the "cavitation or the like") which is caused in the vicinity of the annular recess 12-4. As shown in FIG. 13, moreover, there arise circumferential cracks C.sub.1 which start from the annular recess 12-4 in the joint head 12-2, and axial cracks C.sub.2 which surround the annular recess 12-4.
In order to solve these problems, therefore, we have proposed a method (as should be referred to Unexamined Published Japanese Patent No. 2-57709) for forming the inside of the head into a generally flat circumference by burying a metallic ring member in the head. This method has solved the problem of the cavitation of the like due to the fluid pressure in the joint head 12-2, by forming the annular recess 12-4, as formed in the head by the metallic ring member buried in the head, into the generally flat circumference.
From the standpoint of preventing the air pollution, on the other hand, the fuel for the Diesel internal combustion engine has been exemplified in recent years by dimethyl ether or methanol. However, these fuels are made more corrosive than those of the prior art by the water content or the decomposition products thereof. As the high-pressure metal piping to be used with these corrosive fuels, there has been used a dual pipe in which a thin inner tube made of a corrosive resisting metal such as stainless steel is fitted on the inner circumference of the piping. Alternatively, the main rail 11 itself as the common rail is made of the corrosion resisting stainless steel, or the inner circumferences of the receiving seat 11-3, the branch hole 11-2 and the communication passage 11-1 are plated with a corrosion resisting layer.
When the high-pressure metal piping of the aforementioned dual tube is jointed as the branch tube 12 to the main rail 11 by the fastening nut 14 or 16, as shown in FIG. 14, the inner circumference of the passage 12-1 of the thick steel tube 12' is coated with an inner tube 17 made of the aforementioned corrosion resisting metal, and the pressure seat 12-3 is held in abutment against the receiving seat 11-3 of the main rail 11 made of the corrosion resisting material, so that the inner circumference and the pressure seat have a sufficient corrosion resistance. However, an end face 12" of the joint head 12-2 is exposed at the steel skin of the thick steel tube 12' to the outside so that it is early corroded with the corrosive fuel.